Haven't counted. Also depends how you classify collector items.
List of buyable ones here.
List of collector items and owners. Bit outdated.
Some items like Ishi's Ishi item and other things like beta hairs and other weird and wonderful things aren't there though.

Highest value and lowest value just depends on what someone is willing to pay. Hector relax item is the highest value buyable in the shop due to it's rarity and high $ cost.